In the current state of the art medical devices based on thin film elastomers are manufactured predominantly from vulcanized natural rubber. Usually the film thickness is 3 to 15 mils. Vulcanized natural rubber (VNR) offers particular advantages for protective gloves used by surgeons, dentists and veterinarians who need tactile feel and resilient glove material for their delicate operations. An other class of medical gloves are referred to as "examination gloves." These provide to health care givers and patients protection against viruses and bacterial attack in activities not requiring high level of delicate manual dexterity. Specifically, tactile feel is not an important requirement for examination gloves. In the current state of the art, examination gloves are made of from either VNR or from synthetic rubbers such as nitrile rubber, poly(chloroprene) or plastisized poly(vinyl chloride). In the medical device trade poly(chloroprene) is referred to simply as chloroprene or Neoprene.TM.. Plasticized poly(vinylchloride) is often called "vinyl" in the medical device trade. This terminology will be used herein.
Vulcanized natural rubber is also widely used for tourniquets and dental dams in the health care field. Excellent tactile response and resilience are properties which are crucial for the use of VNR for making condoms.
In summary, synthetic elastomers are useful for examination gloves but, at the present state of the art, do not meet the needs for more delicate medical devices; that is, surgical (including dental and veterinary) gloves, and condoms (for the latter end-uses VNR is almost exclusively used).
The single major shortcoming of VNR is that it causes allergies in a significant fraction of the human population. Reliable statistics are not available but the literature indicates that about 1% of the population is susceptible to the so called "latex allergy."
The major cause of latex allergy is the presence of protein in natural rubber latex. Natural rubber is tapped from rubber trees or rubber plants in form of an aqueous dispersion (latex) and the protein is nature's means to provide colloidal stability to these dispersions. The protein content in VNR can be reduced by using special sources of low protein rubbers or by aqueous extraction. Articles made with low-protein VNR are referred to as hypo-allergenic. The indication is that even minute levels of protein can cause allergies in some people. An objective of the present invention is to provide elastic medical devices which are completely non-allergenic.
Proteins are not the only allergenic ingredients in VNR based devices. For achieving the desired elastic and barrier properties, the rubber must be vulcanized. In this process sulfur-based links are introduced between individual molecules during the curing of the articles. In vulcanization certain accelerators must be used, such as amines, carbothiazoles, sulfonimides, thiocarbamates and thiourams. Some or all of these accelerators are also causes of allergies. In fact, nitrile rubber crosslinked by methods of vulcanizing natural rubber is allergenic though it is completely protein free. The major objective of the present invention is to provide articles made from completely non-allergenic synthetic rubbers crosslinked by completely non-allergenic means.
Most medical devices in the field of this invention are currently made from VNR, chloroprene, nitrile rubber and vinyl. Vinyl is a category by itself. Pure poly(vinylchloride) is not rubbery and is rendered rubbery by plasticizers. Also, the vinyl articles in the field of this invention are most often prepared from water-free plastisols and cannot be made resilient by crosslinking.
Natural rubber, polychloroprene and nitrile rubber are supplied as aqueous polymer dispersions, specifically as latexes. These three latex polymers have a crucially important structural feature in common: they are based on diene monomer building blocks with double carbon--carbon unsaturation. The structure of the dienes is shown below: EQU CH.sub.2 .dbd.CHR--CH.dbd.CH.sub.2
Here R is H for butadiene, CH.sub.3 for isoprene and Cl for chloroprene.
When the dienes are polymerized, each converted monomer retains a single unsaturation as illustrated in the polymer structure below: EQU (--CH.sub.2 --CHR.dbd.CH.sub.2 --CH.sub.2 --).sub.n
Natural rubber is pure polyisoprene. Nitrile rubber is a copolymer of acrylonitrile and butadiene in the 20/80 to 40/60 ratio and is often modified with carboxylic monomers. Polychloroprene is most often made substantially without comonomers.
The unsaturation in the polymer chain is the crosslink acceptor in the vulcanization of natural rubber or nitrite rubber. As stated earlier, the accelerators used in vulcanization with sulfur are allergenic.
The major disadvantage of unsaturation in the diene rubbers is poor stability against heat, radiation and oxidation. Sterilization and packaging after sterilization is rather expensive. The devices made from saturated polymers according to the present invention have the advantage of superior heat, radiation, oxidation and storage stability. A special embodiment of the present invention provides novel inexpensive means for sterilization of medical devices.
U.S. Pat. Nos. 3,872,515 (Mar. 25, 1975, K. G. King et al.) and 5,620,773 (Apr. 15, 1997, B. Nash et al.) teach the use of non-allergenic aqueous silicone based dispersion polymers for making elastic medical devices. The exorbitantly high cost of silicone elastomers precludes their use in the present field of invention.
U.S. Pat. No. 5,500,469 (Mar. 19, 1996, K. E. Johnsen et al.) teaches the use of an aqueous dispersion of copolymer containing hard non-rubbery monomer blocks and diene monomer based rubbery blocks in the same molecule. This polymer is not crosslinked on curing. Instead, the hard blocks provide some resilience to the shaped articles.
Several U.S. patents propose the use of polyurethanes for elastic medical devices.
Two patents teach the application of thermoplastic urethane rubber elastomers to condom shaped mandrels with subsequent curing. In U.S. Pat. No. 4,576,156 (Mar. 18, 1986, M. F. Dyck et al.) the proposed curing temperature is 400 to 450.degree. F. In U.S. Pat. No. 4,684,490 (Aug. 8, 1987, R. A. Taller et al.) the curing temperature is lower, 265 to 350.degree. F. It is also known that the urethane bonds break and rearrange at temperatures above 280.degree. F. leading to crosslinking. These patents do not teach that the urethane rubber be applied from an aqueous dispersion and do not anticipate crosslink formation during heat curing.
U.S. Pat. Nos. 3,813,695 (Jun. 4, 1974, D. L. Dodell et al.), 5,272,771 (Feb. 18, 1992, C. W. Ansell et al.) and 5,534,350 (Dec. 28, 1994, D. Liu et al.) teach lining of VNR gloves with uncrosslinked polyurethanes can be useful either for rendering them hypoallergenic or for improving their donning properties and eliminating the need to use powder to lubricate them internally. U.S. Pat. No. 5,545,451 (Aug. 13, 1996, W. N. Huang et al.) teaches the use of both acrylic latexes and polyurethane dispersions in multilayer VNR based powderless gloves.